1. Field of the Invention
The present invention relates to polyketone fibers and a process for producing the polyketone fibers. More particularly, it relates to polyketone fibers applicable to the fields which require high fatigue resistance, such as tire cords, belts, hoses, ropes, etc., and a process for producing the polyketone fibers.
2. Description of the Related Art
It is known that when carbon monoxide and an olefin such as ethylene or propylene are polymerized using a complex of a transition metal such as palladium or nickel as a catalyst, a polyketone which is a substantially complete alternating copolymer of carbon monoxide and an olefin can be obtained (“Kogyo Zairyo (Industrial Material)”, December, page 5, 1997). Moreover, investigations on use of polyketones as fibers for industrial materials have been made by many researchers, and it is expected to use polyketone fibers in the form of twist yarn cords as fibers for composite materials such as reinforcing fibers for tire cords and belts utilizing the excellent characteristics of polyketones, namely, high strength and high elastic modulus, and dimensional stability, adhesion and creep resistance at high temperatures.
Since a polyketone when molten is apt to be thermally crosslinked, it is preferred to employ wet spinning for making it into fibers. Especially, fibers of a polyketone comprising substantially only carbon monoxide and ethylene (poly(1-oxotrimethylene)) which have excellent mechanical properties are readily crosslinked with heat. Therefore, the fibers are very difficult to produce by melt spinning and they can be obtained substantially only by wet spinning.
When a polyketone is subjected to wet spinning, it is known to use organic solvents, for example, phenolic solvents such as hexafluoroisopropanol, m-cresol and resorcin/water, and resorcin/carbonate (JP-A-2-112413, JP-A-4-228613, JP-A-7-508317). However, fibers obtained by wet spinning using these solvents are readily fibrillated, and are insufficient in fatigue resistance and processability to be used as industrial materials. Furthermore, these solvents are highly toxic and combustible and suffer from the problem that large measures must be taken against the toxicity and the combustibility of solvents for making a spinning equipment of industrial scale.
On the other hand, these methods are proposed which carry out spinning of a polyketone solution prepared by dissolving a polyketone in an aqueous solution containing a zinc halide such as zinc chloride or zinc bromide or a lithium salt such as lithium bromide, lithium iodide or lithium thiocyanate in a specific concentration (WO99/18143, USP5955019). The above aqueous solution is relatively inexpensive, low in toxicity and non-combustible, and are excellent as solvents for polyketones. Moreover, these documents disclose that a strength of 1.5 GPa was attained for monofilaments in a laboratory test. However, according to the tracing experiments conducted by the inventors, when the fibers obtained by using the above solvents are made into the form of multifilaments required for industrial materials, it is difficult to attain a strength exceeding 1.5 Gpa, and, besides, they are insufficient in fatigue resistance and processability to be used as industrial materials. Further, it is difficult to perform stable spinning with less fluffing or breakage.
As a result of investigations conducted on the reasons for the insufficient fatigue resistance and processability of polyketone fibers obtained by the known spinning methods, it has been found that the conventional polyketone fibers have a skin-core structure, in which the surface portions is dense and the central portion is sparse. That is, it is considered that in the case of a structure where the inner portion of fibers being sparse, it is difficult to develop high strength and elastic modulus as fibers, and when the fibers are subjected to processing such as twisting, heat treatment, weaving or knitting, or when they are practically used for industrial materials, the fibers cannot stand compression or repeated extension to cause deterioration of strength or extension, resulting in insufficient fatigue resistance and processability. According to the further investigations conducted by the inventors, it has been found that such skin-core structure is formed owing to the difference in coagulation speed of the surface portion of the fibers and the inner portion of the fibers in the coagulation step. At the coagulation step, the solution of the coagulating bath penetrates into the inner portion of fibers from the surface of the fibers and solidifies the fibers, resulting in difference in coagulation speed between the surface portion and the central portion of the fibers.
Therefore, there is a high possibility of obtaining fibers of high fatigue resistance and processability if polyketone fibers having a uniform and dense inner structure can be produced, but substantially no such investigations have been conducted, and fibers having a uniform and dense inner structure have not yet been obtained.
The first object of the present invention is to provide polyketone fibers having a uniform and dense structure, and, thus, there are provided polyketone fibers which are high in strength and elastic modulus, excellent in fatigue resistance, processability, heat resistance and dimensional stability, and especially suitable for industrial materials. The second object of the present invention is to provide a process for producing the polyketone fibers.